[[p. 361]] It is
odd that there should be a Latin American fauna, a broad unit
that can be roughly designated by such a term as "Latin American," defined
by human linguistics and culture. The animals inhabiting this area can
hardly have foreseen that the dominant languages of the twentieth century
would here be Spanish and Portuguese or that the European cultural elements
imported here would come mainly from Latin Europe--from Spain and Portugal
and also, in considerable measure, from France and Italy. Nevertheless,
there is a characteristic fauna that coincides approximately with Latin
America and that differs in some major traits from the fauna of English-speaking
America. The coincidence is not precise. The fauna of northern Mexico,
although transitional in some respects, is more nearly allied to that
of the United States than to that of most of Latin America. Exact correspondence
of native fauna and imported culture would be a miracle, and it is still
a wonder that the equivalence is as close as it really is. There is, indeed,
a common factor that removes the correspondence from the realm of pure
coincidence. The demarcation both of faunas and of cultures has been influenced
by climatic factors.

Radical differences
in the two major faunal realms of the Americas were noticed by the early
explorers and are still obvious enough to the modern traveler. A New Englander
traveling in Brazil does not need to be a zoologist to observe that the
animals of that country are at least as exotic as are the speech and customs
of its human inhabitants. He will see some animals which, although subtly
different, look reasonably familiar to him: deer, foxes, field mice, squirrels,
rabbits, and a few others. More will be completely new to him outside
of zoos: peccaries, tapirs, jaguars, kinkajous, guinea pigs, agoutis,
capybaras, armadillos, tree sloths, monkeys, and a host of others.

The distinction is not confined to mammals,
a few of which have been named, but extends to all sorts of animals. The
river rays, the lungfish, the piranhas, and many other freshwater fish
are strange to northern eyes. The frogs and toads may look familiar at
first sight, but on closer [[p. 362]] inspection
such forms as the Surinam toad, incubating the young in its back, and,
indeed, almost all others will be found distinctive. Among turtles, the
matamata and other side-necked forms are obviously exotic, and so are
the boa constrictors and numerous other snakes. The ostrich-like rheas
and the wing-clawed hoatzins are only two among the numerous purely Latin
American birds. Additions to the long roster could be provided among earthworms,
insects, and innumerable others.

The fauna is by
no means uniform all over Latin America, but it is evident, first, that
broadly similar faunal characteristics do appear throughout most of this
great area and, second, that these characteristics distinguish the region
sharply from non-Latin North America or any other continent. These facts
led the students of animal distribution in the latter half of the nineteenth
century (the Sclaters, Wallace, Beddard, and Lydekker, to name some of
the more eminent) to set Latin America aside as the Neotropical Region
or Neogaea. In the classical arrangement, this region includes all of
South America (even those parts decidedly nontropical), Central America,
tropical Mexico (but not the temperate central plateau and northern Mexico),
and the West Indies. Although the boundaries are sharp and definite on
the map, it was of course recognized that the line in Mexico is not awesomely
respected by the animals and that it really lies within a broad transition
zone. Some mainly neotropical animals, such as the peccaries or jaguars,
range into the United States, far north of the map line. Some mainly nearctic
(non-Latin North American) mammals, such as shrews, similarly range well
south of the line. It has also long been recognized that the West Indian
fauna is not typically or fully Latin American or neotropical. It is impoverished
relative to the mainland, has its own peculiar forms, and has a few special
resemblances to the North American fauna.

In spite of some
ambiguities, the Neotropical Region does have a well-established clarity
and validity in the zoogeography of the world as it exists today. This
static picture is, however, the result of a long and dynamic historical
process. The nature of the fauna in what is now designated as the Neotropical
Region has changed radically during geological history. Different parts
of that region have not had the same history. Faunal resemblances and
distinctions have sometimes been greater, sometimes less than they are
today. Boundaries of faunal assemblages have not remained in the same
place. Recent neotropical elements may be old in that region or may be
latecomers with their historical faunal associations elsewhere.

Until relatively recently, geologically speaking
(well into the Pliocene), Central America had faunal affinities almost
exclusively with North America and hardly at all with South America. There
was then no Neotropical Region in the present sense. American camels,
tapirs, and other animals now exclusively neotropical were pan-American
in the Pleistocene and exclusively (in this hemisphere) North American
before that. They are surely not neotropical in the historical sense,
or in the same sense as, say, armadillos, which arose in South America
[[p. 363]] and were until the late Tertiary
confined to part of the Neotropical Region as now delimited.

In a historical
view, then, the Neotropical Region ceases to be clear and consistent.
Its fauna is not a coherent unit from this point of view, and its boundaries
are not even approximately constant. In the historical study the static
concept must be abandoned. The region and its fauna cannot be taken as
either definite or invariable, but must be analyzed through a long series
of shifts and fluxes.

Faunal Strata

In any given region,
the various groups of animals will be found to have occupied that territory
for different lengths of time. In general, the longer the group has been
there, the more peculiar it will be to the local scene and the more strongly
differentiated from relatives living elsewhere. The results of this process
are particularly striking in South America. Armadillos and sloths belong
to an order (Edentata) known nowhere else (aside from marginal spread
into North America). Differentiation has proceeded so far in that region
that there is now no clear trace of special relationship to animals of
other regions, aside from the broadest fact that these edentates are placental
mammals. The monkeys of South America are quite distinct, as a group,
from those of the Old World and yet clearly related, belonging to the
same order, Primates. They show an intermediate degree of regional differentiation.
The field mice of South America, although distinctive as to species and,
usually, genera, are closely related to those of North America, belonging
not only to the same order (Rodentia) but also to the same family (Cricetidae)
and even to the same subfamily and tribe. They have a low degree of regional
differentiation.

The fossil record
clearly confirms the fact that these varying degrees of differentiation
are correlated with the time when the ancestors of these animals were
emplaced in South America: edentates, high differentiation, in the earliest
Cenozoic; primates, medium differentiation, in the mid-Cenozoic; cricetids,
low differentiation, in the late Cenozoic. The three groups represent
three readily distinguished faunal strata. The faunal history of South
America has clearly been episodic.

The faunas of all regions are apparently stratified
and their histories episodic in much this same way. On most continents,
however, episodes of emplacement of new groups have been so frequent and
so scattered in time, and their isolation from allied groups of other
regions has been so imperfect, that the stratification is highly complex
and the distinction of various strata is blurred. Only in South America
and in the present island continent, Australia, are the strata relatively
few in number and, as a rule, sharply and clearly separable. This clarity
is a result of the physical history of the continent. Stratigraphic and
paleontological evidence agree in clearly demonstrating that South America
was isolated by sea barriers from all other continents from about the
beginning of the Tertiary until near its end (probably early Paleocene
to late Pliocene). During this long span, on the order of 70 million years,
its [[p. 364]] animals were genetically isolated
from all other continental faunas, and introduction of new stocks was,
although not completely prevented, extremely restricted.

Some complications
exist in each case, but in broad lines the South American mammalian fauna
can be analyzed into three strata. The oldest stratum, which may be called
that of the "ancient immigrants,"

includes groups that reached South America before its complete isolation
from the rest of the world, in latest Cretaceous or earliest Tertiary
times, and the numerous and varied descendants of these ancient stocks.
The second stratum, that of the "old island-hoppers," includes only two
groups, old native rodents and monkeys, which reached South America while
it was isolated. The third major stratum, of "late island-hoppers" and
"late immigrants," with considerable complexity of detail, includes a
[[p. 365]] great variety of animals that
reached the continent in the late Tertiary and the Quaternary, shortly
before and during its re-connection with North America. The three mammalian
strata are shown in more detail in Table I, and their correlation with
geographic events is diagrammatically suggested in Figure 1.

For animals other
than mammals, the fossil evidence is relatively poor. Most of the vertebrates
have more or less clear indications of the same threefold stratification,
with some probable blurring of the picture for groups less rigidly restricted
by a sea barrier (such as some birds and probably some fishes). Among
freshwater and terrestrial invertebrates, the fossil record of which is
almost nil, there may be an added complication in the form of faunal strata
still older than the oldest for mammals and most other vertebrates. On
the whole, however, the

Fig. 1. Relationships between faunal strata
in South America and the rise and fall of intercontinental land connections.
The graph is diagrammatic only, as the history of the connection was not
so simple as shown, and the sea barriers shifted in position.

three designated strata are pervasive in historical interpretation of
the entire fauna.

These faunal strata
can be designated as such and thus dated only with respect to the geographic
unit of South America and by virtue of its Tertiary isolation. In Central
America, for instance, much the same groups of animals occur, but their
time relationships and their associations in strata are quite different
with respect to the region occupied. The oldest mammalian faunal
strata in Central America include what are in South America the old island-hoppers
and various of the late island-hoppers and immigrants. The old immigrants
of South America, forming the oldest stratum there, belong to
what is generally speaking the youngest stratum in Central America.
There are profound differences in regional history in parts of what now
is the broadly unified single Neotropical Region.

[[p. 366]]The Problem
of Basic Mammalian Faunal Types

A fundamental problem
regarding the Latin American mammalian fauna arises at the very beginning
of its history. When the Age of Mammals began, with extinction of many
groups of Mesozoic reptiles and the beginning of the rise of mammals to
dominance in land faunas, the mammals were already, in fact, an old group.
Although still small, obscure, and relatively unvaried, the major lines
of mammals had then already split into two basically distinct, primitive
groups: marsupials and placentals or, technically, Metatheria and Eutheria.

Africa, Europe,
Asia, and North America then formed an essentially continuous land mass,
a World Continent. The continuity has been interrupted at various points
from time to time and there have been innumerable faunal changes and marked
regional differentiation in various groups, but the mammalian faunal type
was then and remains today fundamentally the same throughout all parts
of the World Continent. Marsupials were present at the start (at least
in North America and Europe, and probably throughout), but they were of
minor importance. The major ecological roles, and specifically those of
carnivorous and herbivorous types, were played, as they still are, by
placentals.

In Australia, probably
then already an island continent (Notogaea), and in South America, then
or shortly thereafter becoming an island continent (Neogaea), the basic
faunal types with which the Age of Mammals began were radically different
from the World Continent type. In Australia all the mammalian ecological
roles were played by marsupials. (Such early faunas have not, in fact,
been found in Australia, but this is a probable and generally accepted
inference from the evidence of later faunas.) In South America, the herbivores
were mainly placentals, but the insectivorous, gnawing, and carnivorous
types were marsupials.

Early attempts
to explain this extraordinary anomaly between the South American island
continent mammalian faunal type and the World Continent type usually involved
the theory that South American marsupial carnivores were derived from
Australia. (Why the carnivores of Australia were marsupials rather than
placentals is another fundamental problem not here under consideration.)
Among several serious objections to this idea is the extreme improbability
that marsupial carnivores would spread from Australia to South America
unaccompanied by their ecological associates and prey, the Australian
marsupial herbivores. Other possible sources of mammalian faunas all had
placental carnivores and placental herbivores, and equal difficulty is
encountered in explaining the spread to South America of these herbivores
without their associates, the placental carnivores. That the carnivorous
and herbivorous elements in the first, balanced, basic South American
mammalian fauna should have come each separately and from a totally distinct
source is simply incredible.

An alternative view is that the basic South
American fauna did have an essentially unified geographic origin, all
coming from (or at any rate being connected with) the World Continent
fauna. In that case, [[p. 367]] however,
it would appear that placental carnivores must have been included but
must almost immediately have become extinct in South America. Such extinction
would seem inherently improbable, because in the established cases of
competition between marsupial and placental carnivores, the placentals
have survived and the marsupials have become extinct. The placental dingo
survived in Australia and its marsupial analogue, the thylacine, became
extinct (except on the separate island of Tasmania which was not reached
by the dingo). The last South American marsupial carnivores disappeared
as placental carnivores (of the latest major faunal stratum) arrived.
It is an additional difficulty that specialized marsupial carnivores are
quite unknown from any age on the World Continent and probably never occurred
there. Similarly, placental carnivores are totally unknown in the early
South American faunas, and extinction so rapid and complete as to eliminate
all traces in the now rather abundant early Tertiary fossil collections
seems improbable, although not impossible.

These objections
largely disappear, and a reasonable solution of this long-disputed problem
can be suggested, if one gives consideration to the relatively undifferentiated
and primitive mammals of the earliest Tertiary, rather than thinking in
terms of the more familiar, sharply distinct and specialized later forms.
The basic World Continent fauna, at its very beginning, did not include
radically differentiated placental carnivores and herbivores. These two
later ecological types were then ancestrally represented by one basic
stock, to which I have elsewhere applied the name "ferungulate." This
stock was rapidly becoming considerably varied and apparently included
some rather more herbivorous and some rather more carnivorous forms, but
as a whole it was omnivorous and all its members were still closely similar.
Among the fossils of this group, some are classified as Carnivora and
some as Condylarthra (primitive ungulates), because of our knowledge of
the subsequent sharp ordinal distinctions of their descendants.
If we did not know their later history, the early forms would certainly
be placed in the same order and very likely in the same family or even
smaller group. For instance, Protogonodon, an early Paleocene
form classified as a carnivore, and Desmatoclaenus, also early
Paleocene, classified as an ungulate, are far more alike than numerous
pairs of recent genera referred to one family or subfamily, say Mustela,
the weasel, and Gulo, the wolverine, both commonly placed in
the subfamily Mustelinae.

It was this more
generalized ferungulate stock, and not any later and then truly distinctive
placental carnivores and herbivores, that figured in the beginnings of
the oldest faunal stratum of South America. On the World Continent, this
stock was associated with likewise primitive marsupials, which also lacked
as yet any distinctive specializations as carnivores but were equally
capable (as the event proved) of such a development. In the World Continent,
after it was cut off from South America, specialized carnivores
arose among the ferungulate lines, and their occupation of this ecological
specialty impeded the rise there of marsupial carnivores. In South America,
likewise after separation [[p. 368]] from
the World Continent, marsupials more rapidly developed carnivorous types.
This in itself would tend sufficiently to inhibit the rise there of placental
carnivores among the ferungulates, which rapidly developed a great variety
of types all more or less herbivorous.

Thus there was
no competition between marsupial carnivores and placental carnivores,
as such, and probably only marginal competition between marsupials and
placentals as a whole, but only a parceling out of the various ecological
zones, which happened to receive different occupants in the World Continent
and in South America. Why the ferungulates evolved more rapidly into specialized
carnivores on the World Continent and the marsupials (didelphoids) in
South America is not evident. It is, nevertheless, plausible that this
could occur, and the hypothesis that it did occur seems to be the only
one that reasonably accounts for these differences in basic faunal type.

There may be some
remaining objection that placentals in general are superior to marsupials
and would tend, in South America or elsewhere, to occupy any and all ecological
niches, including those of carnivores, at the expense of the marsupials.
This possible objection really has little or no force. Present evidence
is that the placentals are not more advanced derivatives from backward
or older marsupials, as was commonly believed in the nineteenth century
and is still occasionally stated in textbooks. Placentals and marsupials
seem rather to represent a basic dichotomy of the main mammalian stock
and to have been about equally progressive and adaptively efficient when
they arose. Primitive forms of the two major branches apparently lived
together in ecological equilibrium, and even today the opossums of North
and South America remain abundant and are eminently successful in holding
their own in the midst of the dominant placental faunas. Ecological incompatibility
between marsupials and placentals seems to arise only when late and narrowly
specialized forms come in contact within essentially the same ecological
niche, a situation in which any two groups, and not only marsupials
and placentals, become ecologically incompatible. Even on this score,
the extinction of South American and Australian marsupial carnivores in
competition with placental carnivores gives evidence not particularly
that placentals are superior to marsupials but that the late World Continent
groups had become competitively superior to those of the island continents.
In South America, the old native placental groups were also decimated
when they came into contact with placentals from the World Continent.
They fared no better than the marsupials of similar geographic history.

Origins of the Oldest Faunal Elements

Probably the most disputed single question of
Latin American zoogeography has been the geographic origin or, at least,
connections of the older faunal elements. The principal question raised
has concerned possible direct relationships among the southern continents,
between South America and Australia, on one side, or Africa, on the other.
The various postulates include transoceanic land bridges or continents,
[[p. 369]] land connections by way of Antarctica,
or early continental union followed by fracturing and drift (Wegenerian)
to the present widely separated positions of the three continents. A summary
of the truly voluminous and polemic literature of this subject is outside
the scope of

Fig. 2. Marsupials and edentates, evolved in
the oldest mammalian faunal stratum of South America, and their possible
origins. (Not drawn to scale.)

the present study, but the nature of the older mammalian faunas and the
evidence these give as to their origin will be briefly reviewed.

The major elements
among the mammalian old immigrants and their possible origins are as follows
(see also Figs. 2 and 3).

[[p. 370]] MARSUPIALS.
These were already quite varied in the older Tertiary, with primitive,
more or less insectivorous types, rodent-like types, and marsupial carnivores.
A possible common origin of all would be in a varied assemblage of relatively
unspecialized marsupials, didelphoid or

Fig. 3. Ungulates of the oldest mammalian faunal stratum
of South America, and their possible origins. (Not drawn to scale.)

extremely primitive dasyuroid in general character. Such an assemblage
is known from the late Cretaceous of North America, where primitive marsupials
(opossums) also occurred through the Paleocene and later. Similar forms
occur doubtfully in the Paleocene and surely in the Eocene of Europe.
Elsewhere the evidence is purely negative.

[[p. 371]] EDENTATES.
The oldest South American faunas include only armadillos, of primitive
type, among edentates. The other groups appear, evidently by evolution
within the continent, during the early and middle Tertiary: glyptodonts
and ground sloths by late Eocene, anteaters in the Miocene, tree sloths
(with practically no established fossil record) by rather minor differentiation
from the less specialized ground sloths sometime around the mid-Tertiary.
The only firmly established special relationships with any non-South American
mammals are with the Palaeanodonta, a group more primitive in general
character than any of the South American edentates and known only from
North America (late Paleocene to middle Eocene, with a probable more specialized
offshoot in the early Oligocene).

CONDYLARTHS. These
most primitive of ungulate herbivores were fairly common in the Paleocene-Eocene
faunas of South America and some lingered on into the Oligocene (or possibly
the Miocene). The same order was important in the basic World Continent
fauna. It occurs abundantly throughout the Paleocene and Eocene of North
America and some North and South American genera are suggestive of special
relationship.

LITOPTERNS. This
order, known only from South America, was common and varied in early faunas
and persisted, in decreasing variety, into the Pleistocene. Early forms
are near the condylarths, to such an extent that the litopterns might
be considered merely as surviving and diversely specialized condylarths.
They seem to have originated in South America from the South American
condylarths, and therefore to have the same source as the latter.

NOTOUNGULATES.
This great order of hoofed herbivores constitutes the bulk of all the
earlier South American faunas and continues, with radically decreasing
variety, into the Pleistocene. It suggests a group similar in origin and
parallel in history to the litopterns. An odd anomaly arises from the
discovery of a notoungulate in the late Paleocene of Asia and a related
form in the early Eocene of North America, the only occurrences of the
order outside of South America. Origin in Asia and migration to South
America by way of North America are suggested by the face of these facts,
and this remains a possibility but is rendered rather improbable by various
considerations. Early Eocene is too late a time for entry into South America.
The most primitive South American forms are slightly but definitely less
specialized than those known from Asia and North America. Another possibility
is that the latter were strays from South America. The question
cannot now be resolved, but a faunal connection of one sort or another
is indicated with the northern continents.

ASTRAPOTHERES. Although never highly varied,
these aberrant hoofed herbivores appeared early in South America and persisted
into the Miocene. They probably arose in that continent and have no evident
close or special relationships with other known groups, except that they
probably originated ultimately from condylarths or most primitive ferungulates
in a broader sense. They could have some collateral [[p.
372]] relationship or, at least, functional parallelism with archaic
World Continent forms like the pantodonts and uintatheres, known in particular
abundance from North America but also spreading to Eurasia.

PYROTHERES. These
odd, superficially mastodon-like ungulates were never much varied or particularly
common, as far as known, but they appear in early Eocene faunas and persist
into the Oligocene. They are known only from South America and probably
arose in that continent. Special relationships are not known, but in broad
terms differentiation from the basic ferungulate complex is indicated,
more or less as for the astrapotheres. True relationship to the Proboscidea
has been suggested but now seems untenable except as the Proboscidea may
also represent a separate branch from a part (paenungulate) of the ferungulate
complex.

The data on the
very earliest Tertiary mammals of South America and of the world as a
whole are so scanty that tracing of exact lineages cannot be expected
and is not, in fact, possible at present. In broad terms, this assemblage
certainly suggests derivation from the World Continent fauna and could
not be derived from Australia unless it is assumed, quite gratuitously
and contrary to such evidence (all indirect) as exists, that Australia
did once have a basic fauna of World Continent type and later eliminated
almost the whole of this fauna. Within the World Continent, there is absolutely
no evidence of any special relationship of Africa with South America.
This is partly a negative conclusion, because Paleocene mammals are unknown
from Africa, but it would, again, be wholly gratuitous to assume that
relationships existed and left no trace in the late Eocene and subsequent
African faunas.

Such definite evidence
of relationships as exists all points most nearly to North America. Every
South American stock is related at least as closely to one known from
North America as to any other known group outside of South America, and
the edentates have no established relationships except with an exclusively
North American group. In view of these relationships and of the fact that
South America is now geographically closest to North America (and nothing
impels the belief that this was untrue at the end of the Cretaceous),
the tendency of so many students to look elsewhere for the main geographic
relationships of the old South American fauna almost smacks of a preference
for the unlikely over the obvious.

The question remains whether the World Continent
fauna might not have been derived from South America, as the great Argentine
student, Ameghino, insisted, rather than the other way around. In the
exact sense of Ameghino, who believed that later specialized orders and
families originated in South America and spread hence over the rest of
the world, this certainly is not true. In the sense that some elements
of the most basic World Continent fauna might have become differentiated
in South America while that continent was united to North America and
hence in a sense part of the World Continent, the possibility remains
but it seems quite improbable. The old South American faunas seem to represent
diversification on that continent from a sampling of the World Continent
fauna that was partial only (no insectivores, no early primates, no early
[[p. 373]] rodents, no differentiated creodonts,
only highly aberrant paenungulates). No forms conceivably near the ancestry
of any World Continent lineages have ever actually been found in the old
South American faunas.

Clearly inconclusive,
the evidence does suggest North America as the probable source of the
old South American fauna. It does not exclude, but definitely does not
support, other possibilities. This conclusion finds further, indirect
but strong substantiation from the evidence that South America was connected
with North America toward the end of the Age of Reptiles. Although opposite
statements may still be found in the literature, it is at present recognized
by the most competent specialists on these faunas that the late Cretaceous
reptiles of South America are, as a group, more like those of North America
than like any known faunas elsewhere in the world.

The Old Island-Hoppers

The origin of the
second faunal stratum of South America, although less discussed as a separate
problem, has been in many ways even more puzzling. This stratum comprises
the old native rodents, which are the so-called South American hystricomorphs,
and the neotropical primates (see Fig. 4). These rodents first appear
in the record in beds somewhat uncertainly correlated as early Oligocene,
and primates are first known from later beds, considered with similar
uncertainty as late Oligocene. Both occurrences are marginal on the continent
(in Patagonia) and do not exclude the possibility that the groups had
been for some time in more central or northern parts of the continent,
where adequate faunas of appropriate age have not been discovered. It
is, however, improbable that emplacement of these groups in South America
was long prior to the Oligocene and it seems almost certain that their
ancestors were not among the early immigrants. There is no evidence that
entry of the two groups was absolutely simultaneous, and, indeed, it probably
was not. Both, however, entered South America at about the same time,
roughly midway between the two major immigrations; the two have become
comparably differentiated there, and it is justified to consider them
broadly as of the same faunal stratum.

Both these groups
have repeatedly been cited as indicating faunal relationships with Africa.
The neotropical monkeys, although distinctly definable as a group, resemble
the Old World (including African) monkeys and are on a similar evolutionary
level. The old South American rodents also show resemblances to some Old
World rodents (porcupines figure in both regions, for instance), and among
them are some, in the general group of the hutias, degus, tucutucus, and
spiny "rats" (not true rats), that particularly resemble some African
rodents, the cane and rock "rats" (also not true rats).

Recent studies seem rather conclusively to controvert
these apparent African affinities. The South American primates, including
the earliest forms, are in some respects more primitive than the Old World
forms of similar or later age and in some respects differently specialized.
The [[p. 374]] former characters seem to
prohibit derivation of the New from the Old World forms, and the latter
seem to exclude filiation in the other direction. There is a strong suggestion
that the New and Old World monkeys

Fig. 4. Derivatives of island-hopping immigrants into
South America, and their probable origins. The monkeys and old native
rodents form the second or intermediate mammalian faunal stratum. The
procyonids (allies of the raccoon) may be considered rather as forerunners
of the last, relatively complex faunal stratum. (Not drawn to scale.)

represent geographically separated parallel developments from a more
remote and primitive (technically prosimian) ancestry. About twenty-five
years ago, the late J. W. Gidley suggested that neotropical monkeys [[p.
375]] might be derived from a group of prosimians (Notharctinae)
relatively abundant in the Eocene of North America, and the Old World
monkeys from Old World allies of this group. Too little attention has
been given to this suggestion, but recent study is adding some evidence
in its favor and it is now the best working hypothesis as regards the
New World forms, at least.

It has been suspected
from time to time that the history of the New and Old World "hystricomorph"
rodents might be a similar case of independent, parallel development from
allied New and Old World groups of primitive Eocene rodents, but concrete
evidence has been scanty. Quite recently (in 1949), in describing relatively
complete material of the oldest and most primitive known South American
rodent, A. E. Wood has found positive evidence for their derivation not
from a distinct hystricomorph stock but from a widespread World Continent
group of most primitive rodents (Ischyromyidae, sensu lato).
Among these, he finds particular resemblance to some North American forms
(especially the mid-Eocene Reithroparamys).

There is, then,
good reason to believe that the idea of special African relationships
for these groups, old South American rodents and monkeys, is incorrect
and that both are of North American origin. The evidence is, indeed, better
than for the older immigrants, for in each case a possible North American
ancestry can be rather closely designated among well-known groups.

It seems quite
clear that these groups did not follow a land bridge from North America.
When they entered South America, North America was swarming with rapidly
progressive mammals of other types, notably many placental carnivores
and a variety of perissodactyl and artiodactyl ungulates. It is incredible
that an open migration route existed without any effect other than spread
southward of just two stocks, possibly only a single introduction in each
case. Entry was almost certainly by waif dispersal over what I have called
a "sweepstakes route." Both groups, initially small and probably arboreal
animals, are ecological types especially apt for such dispersal. Geological
evidence in Central America and northwestern South America perfectly fits
this picture. During late Eocene and Oligocene, the pertinent times in
this connection, there was clearly a series of seaways between North and
South America, interrupted by a series of islands. The seaways would bar
any extensive faunal interchange. The islands would facilitate overseas
spread of a few special groups, literally island-hoppers. Although it
would, of course, be possible to postulate a similar island chain elsewhere,
to Africa, for instance, no evidence known to me really suggests this
and the postulate is unnecessary and unsupported.

The immediate source
of the immigrants would, of course, be Middle America or what is now tropical
North America, and not the region from New Mexico to Montana where early
Tertiary faunas are now known. Transition from the archaic ancestral groups
(prosimians for the monkeys and protrogomorphs [of Wood] for the rodents)
to the characteristic Latin American groups (ceboids and "hystricomorphs"
or, perhaps [[p. 376]] better, caviomorphs,
respectively) doubtless occurred in the paleontological terra incognita
of early Middle America. Some basic diversification of these groups may
also have occurred in Middle America and have been under way even before
they island-hopped to the southern continent.

Development of the Native Fauna

The oldest reasonably
well-known South American mammalian faunas (Eocene of Patagonia) are dominated
by the typical and highly varied native ungulate herbivores of the Order
Notoungulata, which includes nearly half of the known genera of that time.
The rest of the known fauna is divided about equally among condylarths,
litopterns, edentates, and marsupials (groups characterized on previous
pages). Pyrotheres and astrapotheres were present and are striking animals,
but were quite minor elements in the total fauna.

Until the late
Pliocene, the faunal composition changed rather steadily but with few
really profound modifications. The condylarths dwindled and finally became
extinct, with ecological replacement by their collateral descendants,
the litopterns. The notoungulates, exuberantly varied in the Eocene, continued
in force but show a steady decrease in variety as primitive and intermediate
lines were weeded out and a smaller number of more sharply distinct and
specialized lines continued. On the other hand, the edentates, relatively
little varied in the Eocene, expanded steadily into the Miocene when,
in variety of genera, they constituted about a third of the known faunas.
Pyrotheres and astrapotheres died out, the former in the Oligocene and
the latter by the end of the Miocene.

The most noteworthy
change before late Pliocene followed the appearance of the second faunal
stratum, that of the old island-hoppers. The old native rodents expanded
steadily and greatly into the Pliocene, where they include about a third
of the known mammalian genera. Similar, but less intense, expansion of
the primates may be postulated, although they are absent in most of the
known fossil deposits, which generally represent facies unsuitable for
this predominantly tropical and arboreal group. The broader lines of this
long faunal development are seen in Figure 5.

The greatest interest
of the phase of South American faunal history while the continent was
an island lies in the fact that it is a sort of large-scale natural experiment
in evolution. The old immigrants rapidly occupied the large and varied
continent, and the faunas then evolved there in dynamic equilibrium, undisturbed
by wider genetic interchange or by irruptions from without other than
those of the old island-hoppers--and this exception is itself so relatively
simple and analyzable that it almost ideally exemplifies the process and
consequences of single major additions to an evolving fauna.

The first striking
evolutionary phenomena illustrated are those of "explosive" or "eruptive"
evolution and adaptive radiation. The poorly known Paleocene faunas and,
particularly, the better known early [[p. 377]]
Eocene faunas are at a most active stage of this process. Everything here
indicates the culmination of an exceptionally rapid and burgeoning expansion
of the mammalian fauna into a great variety of ecological niches which
were, before this episode, empty or nearly so. The number of separate
groups, from generic to family levels, at least, is exceptionally large.
Intergradation between lines later widely distinct is still common. There
seems to be considerable overlapping and even duplication in ecological
types within the same or adjacent local faunas. Variation within specific
populations is often exceptionally great, a basis for rapid diversification
speciational in pattern, and an indication that marked segregation of
characters and specialization of adaptive type are under way but still
incomplete.

Disappearance of
intermediate types and fixation of a smaller number of well-defined groups
each with a characteristic, separate, and

Fig. 5. Development of the land mammals of South America.
For each epoch, the composition of the fauna is shown in terms of percentage
of known genera belonging to the various orders of mammals.

progressively specialized adaptive and ecological status were processes
evident through the later Eocene and Oligocene. This process was essentially
completed in the Miocene as far as the descendants of most of the ancient
immigrants are concerned, although the old native rodents and some of
the edentates were then still in an expanding phase. The result of the
process is a sort of parceling out of the available ways of life among
the various stocks of mammals, an adaptive radiation.

In the meantime, on the World Continent even
more varied ways of life were being occupied by adaptive radiation, or
a more complex, interlocking series of over-all and local adaptive radiations,
going on without contact with the South American fauna. When World Continent
and South American lines became specialized for similar ecological roles,
they also came to resemble each other functionally and morphologically.
[[p. 378]] Parallel and convergent evolution
was thus illustrated on a large scale. Another independent radiation in
Australia produced another set of parallel and convergent types. Sometimes
the separate lines departed from the same more or less remote common ancestry
and evolved in close parallel, as in the case of the carnivorous marsupials
of South America and of Australia. This process produces closest resemblance,
a fact responsible for long debate on the affinities of these marsupial
carnivores and for former insistence on the part of some students that
South America and Australia must somehow have shared advanced and specialized
marsupials and not merely the remote and primitive ancestors of these.

In other cases
quite different groups have evolved toward similar adaptive types, a process
producing less complete, convergent resemblance, as between the marsupial
carnivores and the World Continent's placental carnivores. The two processes
of parallelism and convergence intergrade and cannot be sharply defined
in given cases. The striking similarity of some North American horses
and some South American litopterns, evolving independently from a more
or less remote condylarth ancestry, is a case in point. It can be interpreted
either as straight parallelism from the common ancestral condylarth stage
or as divergence, origin of the different orders Perissodactyla and Litopterna,
followed by convergence between lineages in two families of these orders,
Equidae in the first and Proterotheriidae in the second.

The faunas well
illustrate the limitations of these processes, which produce similarities
of various degrees but apparently never, even in cases of close parallelism,
produce real identity, in part or in whole. In the flesh, a superficial
or distant observer might well have confused Diadiaphorus, a
South American litoptern, with Miohippus, a North American horse,
but no competent anatomist would mistake any tooth or bone of one for
the other.

Another interesting
point is that such developments were not necessarily synchronous in the
two cases. Thoatherium, a litoptern, was completely one-toed
in the early Miocene. Horses did not become one-toed until the Pliocene,
and even today Equus is less advanced than Thoatherium
was in this respect (see Fig. 6). Some lines of notoungulates in South
America had very high-crowned, complexly crested, cement-covered grazing
teeth in the early Oligocene; horses did not reach a comparable stage
until the late Miocene. These examples illustrate, by the way, that South
American animals were not altogether less progressive or more slowly evolving
than those of the World Continent, as has sometimes been supposed.

In many cases convergence was quite incomplete
or would involve only a particular functional resemblance and not a close
equivalence of total ecological status. Glyptodonts, for instance, the
rigidly bulky cousins of the armadillo, seem to have been grazing forms
and so have this functional resemblance with various ungulate grazers,
but even their teeth are built on a plan wholly different from that of
any ungulate. The glyptodonts as a whole can hardly be compared with any
ungulate, [[p. 379]] or indeed with any World
Continent animals. Such forms as some litopterns and some horses may be
considered ecological vicars in their respective areas, but others, like
the glyptodonts, are ecological uniques.

Late Faunal Mixture and Its Outcome

In the World Continent,
there was a radical turnover in faunal type rather early in the Tertiary,
mainly during the Eocene. In terms, for instance, of percentage composition
of the mammalian faunas by orders or suborders, the difference between
a Paleocene and an Oligocene fauna in North America is striking and almost
absolute. Changes in this respect, of broad lines of faunal composition,
have been relatively slight since the Oligocene. The modern faunal type
was beginning to appear and to replace the oldest type even in the Eocene,
thus here aptly called "Dawn of the Recent." Not so, however, in South
America. There, as we have already seen in passing, change was gradual
and involved no really fundamental upset of faunal type into the Pliocene.
The mid-Pliocene fauna, in most of its broadest features, was not radically
unlike a Paleocene fauna in spite of very pronounced advancing specialization
in most of the orders and the Oligocene insertion into the fauna of two
new orders (Primates, Rodentia).

Fig. 6. Side views of hind feet of the modern horse,
Equus (left), and of an extinct South American pseudo-horse,
the litoptern Thoatherium (right), to show convergence
in foot structure of these two one-toed mammals. Note vestiges of side-toes,
larger in the true horse than in this pseudo-horse. (Not drawn to scale;
Equus is larger than Thoatherium.) Redrawn after W.
D. Matthew.

A change like that going on in the Eocene on
the World Continent also occurred in South America, but at a greatly later
date, in the late Pliocene and Pleistocene (Fig. 5). Its cause was the
rise of the Central American bridge and the consequent irruption into
South America of many derivatives from the fauna of the World Continent.
This third broad faunal stratum did not come in all at once, in a single
wave. Already in the late Miocene a few northern forms appeared, small
arboreal placental carnivores more or less related to the raccoon. Not
long thereafter, apparently in early Pliocene times, some South American
animals, ground sloths, reached North America. These forerunners do not
seem to indicate a continuous land connection but probably utilized [[p.
380]] the island chain, gaps in which were closing progressively
as the Central American and northwestern South American regions rose relative
to sea level. The exact moment when the bridge became complete is not

FIG. 7. Representatives of the principal families of
previously North American mammals that invaded South America late in the
Cenozoic, and which form the youngest mammalian faunal stratum there.
(Not drawn to scale.)

[[p. 381]] established, but this probably
occurred during the age called Chapadmalalan in South America and Blancan
in North America, placed by some authorities as latest Pliocene and by
others as earliest Pleistocene. Even then the exchange was at first rather
limited in scope and the full surge of intermigration did not occur until
somewhat later, in unequivocally Pleistocene times. Soon or late, at least
fifteen (possibly sixteen) families invaded South America in this great
episode (see Fig. 7).

Invasion occurred
in both directions. By a moderate tabulation, fifteen families of North
American mammals then spread into South America and seven families spread
in the reverse direction. The main migrants to the south were rabbits,
squirrels, field mice, dogs, bears, raccoons, weasels, cats, mastodons,
horses, tapirs, peccaries, camels, and deer, including in most of these
cases some variety of related forms.

Fig. 8. Development of some South American groups that
were ecologically unique with respect to late invaders from North America.
Height of the graph in each case represents the relative number of known
genera. The time scale at the bottom applies to all graphs. NA = migration
to North America. (Some of the old native rodents were also ecologically
unique; see Fig. 9.)

The immediate effect
was to produce in both continents, but particularly in South America,
a greatly enriched fauna. To a fauna already large and essentially complete
or closed ecologically were added a large number of new forms from the
other continent. The enrichment inevitably involved some duplication.
No two forms of different origin can have been precisely and fully equal
in their needs and capacities, but many were sufficiently similar to be
in competition for food and, in general, living space. Some native groups
held their own and some invading groups became extinct, but some native
groups disappeared and (as a rule) were replaced by invaders.

In the end, that
is, at the present time, South America has returned to about the same
basic richness of fauna as before the invasion. Recent families of mammals
there number the same (more or less, depending on the classification used)
as in the Pliocene before invasion, but the faunal composition is radically
different. The notoungulates, litopterns, and marsupial carnivores have
entirely disappeared. The native rodents and edentates are greatly reduced.
In their places, artiodactyls, perissodactyls, rodents of northern origin
(squirrels, cricetids), rabbits, and placental carnivores are fully entrenched
and constitute, in number of genera, about half of the recent fauna.

[[p. 382]] The
main determinants in this process were, first, ecological status and,
second, place of origin. Ecological uniques tended to survive (Fig. 8).
It is true that glyptodonts and ground sloths, after spreading over both
continents, became extinct and that they apparently were ecological uniques.
The question of their extinction involves some other and not properly
understood factor, and it is part of a larger question that cannot be
discussed here. The smaller, likewise or even more strictly ecologically
unique relatives of these animals, armadillos, tree sloths, and anteaters,
did survive. So did the monkeys, ecological uniques with respect to North
America, and most of the truly ecologically unique old native rodents.

Fig. 9. Development in South America of ecological vicars,
similar in adaptation to invading types from North America. Height of
the graph in each case represents the relative number of known genera.
The time scale at the bottom applies to all graphs. Top and bottom graphs
show simple, total replacement of old groups by late invaders from North
America. The more complex middle graph shows total replacement of old
rodent-like marsupials (Polydolopidae) by other old native rodent-like
forms, partial replacement of old rodent-like ungulates (mainly typotheres
and hegetotheres) by old native true rodents, and then partial replacement
of most of the latter by rabbits and true rodents in the late invasion
from North America. Surviving old native rodents are, in the main, ecologically
unique, with no closely similar competitors among the late invaders. *
= apogee.

When ecological
vicars met, one or the other generally became extinct (Fig. 9). In South
America, old native ungulates disappeared and ungulates of northern origin
survived. Many old native rodents (about half the generic lines) became
extinct and rabbits, squirrels, and field mice survived. Marsupial carnivores
became extinct and placental carnivores survived. The fact that in each
case the survivors were of northern origin cannot be pure coincidence.
It is not explanatory to say that the animals from North America were
"superior" or "more progressive," and such statements would be hard to
substantiate by any objective evidence from their anatomy, for instance.

The ultimate factors have not been and probably
cannot be designated, but a generalized explanation presents itself. North
American animals had intermittently throughout the Age of Mammals and
almost continuously in its later part been involved in the flux and intermigration
of the World Continent. Those extant in the Plio-Pleistocene were the
ones that had been successful in a long series of competitive episodes.
They were specialists in invasion and in meeting competitive [[p.
383]] invaders. South American mammals had competed among themselves
in the early Tertiary, but by about the end of the Oligocene they had
essentially completed a process of parceling out the ecological opportunities
among a number of practically noncompeting lines. Thereafter until the
late Pliocene they met no impact from outside their own closed economy,
and when it came, they had not evolved the required defenses.

Thus the recent
South American mammalian fauna is a complex agglomeration, in spite of
the fact that processes of ecological adjustment have again reduced it
to an essentially balanced economy. It may all be ultimately derived from
North America, and some of the evidence to that effect has been summarized.
This evidence is suggestive but inconclusive for the oldest elements,
stronger for the mid-Cenozoic elements, and conclusive and unquestioned
for the latest elements. Even if all did have the same geographic origin,
there are the three quite distinct broad faunal strata. Their different
times of entry into South America complicate the picture, not only because
of corresponding differences in differentiation within that continent,
but also because each stratum was drawn from a different sort of World
Continent fauna. Differentiation, replacement, intermigration, complex
stratification, divergent specialization, and other processes were constantly
going on in the World Continent, and the strata of South American mammals
sampled this sequence at three very different levels.

The static zoogeographic
picture in a classical sense, of current resemblances and differences
of regional faunas, is further complicated by differential regional extinction.
Pumas link North and South America in nonhistorical zoogeography because
they now occur on both continents, and camels separate the faunal regions,
because they occur in South but not in North America. But in the Pleistocene
pumas and camels occurred throughout both continents and in the Pliocene
their ancestors were present in North but not in South America. Final
analysis of the present relationships of South and North American faunas
must involve this factor as well as the factor of stratification.

In Table II an
attempt is made to analyze the now extant South American mammals with
respect to their geographic history and their present relationships to
mammals in the recent North American fauna (north of the tropical zone).

This analysis is
on a family level. More refined analysis, to genera or below, would involve
some changes of status in the last stratum. Central America, with its
peculiar faunal history and its intermediate faunal types, is not involved
in this comparison, which is based on the fully South American and the
temperate-zone North American faunas. Special consideration of Central
America is now necessary.

The Role of Central America

There is and doubtless always has been
considerable difference between regional faunas within South America.
The faunas of the Patagonian pampas, the Andine punas, and the tropical
rain forests are strikingly [[p. 384]] distinct.
They are, nevertheless, regional varieties or differentiates of a general
South American fauna and they share much the same sort of differences
from and resemblances to the general fauna of temperate North America.
There have not, during the Cenozoic, been any absolute barriers between
regions within South America, and no outstanding effects of isolation
are historically evident in separate parts of the

TABLE II--ANALYSIS OF ZOOGEOGRAPHIC RELATIONSHIPS OF SOUTH
AMERICAN RECENT MAMMALS, WITH RESPECT TO NORTH AMERICA

I. Oldest South American faunal stratum, from North America
(?) in late Cretaceous-Paleocene.
A. Collaterals (strongly differentiated)
from same level present in North America.
1.
Without late level spread and recent presence in North America: caenolestids
(North American Colaterals: opossums). (Some students would place the
caenolestids under IB1 and the opossums under IB2; this may be correct
but seems unlikely at present.) 2.
With late level spread, etc.: none.
B. Ancient stock extinct in North
America.
1.
Without late level spread and recent presence in North America: none.
2.
With late level spread, etc.: armadillos (N. A. distribution restricted).
II. Intermediate South American faunal stratum, from North America in
Oligocene.
A. Collaterals (well differentiated)
from same level present in N. A.
1.
Without late level spread, etc.: "hystricomorphs" or caviomorphs except
porcupines (differentiated N. A. collaterals: sewellel, squirrels, etc.).
2.
With late level spread, etc.: porcupines.
B. Ancient stock extinct in North
America.
1.
Without late level spread, etc.: monkeys. (No collaterals in N. A. wild
fauna, but man could be considered as such.)
2.
With late level spread, etc.: none.
III. Late South American faunal stratum, from North America in late Pliocene
and Quaternary.
A. Collaterals from same level (here
poorly differentiated and representing essentially the same immediate
stock) present in N. A. Late level spread back to N. A. possible in some
cases, but in that event part of the same general episode of intermigration
as emplacement of the South American stratum: opossums (history somewhat
questionable, sometimes considered IB2, but probably belonging here),
shrews (marginal and with slight penetration in S. A.), rabbits, squirrels,
cricetids, dogs, bears, raccoons, weasels, cats, peccaries (marginal and
with slight penetration in N. A.), deer.
B. Collaterals extinct in N. A.: tapirs,
camels. (Peccaries, listed under A, are marginal between A and B.)

continent. At most there has been a sort of climatic zoning by which
animals once more widespread have become confined to particular, areally
definable environments (primates to tropical forests, camels to mountains
and cold plains, etc.), or the faunas of deteriorating environments have
been progressively impoverished (in Patagonia, for example). Mid-Cenozoic
faunas lately found in Colombia (cooperatively by the [[p.
385]] Colombian government and the University of California, under
the direction of R. A. Stirton) have interesting regional differences
from contemporaneous Patagonian faunas, but are definitely of the same
general type and reveal no unexpectedly exotic groups. An old idea that
northern and southern South America were separated by a Tertiary sea barrier
and had quite distinct faunal histories, an idea still current among a
few students but long rejected by others, is thus conclusively proved
false.

There are, nevertheless,
two regions of (broadly speaking) Latin America and of the Neotropical
Region of static zoogeography that have had faunal histories decidedly
different from that of South America: Central, or in a somewhat broader
sense Middle, America and the West Indies. The development of the West
Indian fauna is one of the most fascinating topics of historical zoogeography
and has been the subject of long, sometimes bitterly polemic discussion
involving fundamental principles of this science. It is, however, of minor
importance for the present broader theme and cannot be considered here.
The West Indies have been a faunal dead-end. There is little evidence
that they have had a reciprocal influence on the larger faunas of the
continental mainlands.

Middle America, on the
contrary, has had an essential and striking role in the development of
the faunas of all the Americas. A few admirable studies of the static,
modern zoogeography of parts of the region have been made, and some attention
has been given to historical aspects, but full evaluation is lacking and
too little thought has been given to this subject. A cause for relative
neglect has been the extreme paucity of primary historical documents.
With one possible exception of no present value, no pre-Pliocene nonaquatic
fossil has ever been found in tropical Middle America. There is one known,
rather small but extremely important, early Pliocene mammalian local fauna,
from Honduras (found and described by P. O. McGrew and associates), and
there are several scattered Pleistocene faunules, of considerable interest
but still lacking forms (especially the smaller mammals) that would be
more fully enlightening.

Direct and conclusive
study will require a good sequence of Central American land faunas, including
vertebrate microfaunas, from Miocene, at latest, to recent. There is no
assurance, nor even any considerable probability, that such a sequence
remains to be discovered. It is nevertheless possible to evaluate the
role of this region with some assurance, even if without adequate detail,
on the basis of the scraps of local, direct evidence and the increasingly
imposing array of indirect evidence.

The recent fauna of Central
America is essentially like that of South America. The same faunal strata
occur there, although, as already noted, their ages and relationships
with respect to the area occupied are here different. Environmental conditions
in Central America are similar to those of adjacent tropical South America.
With union of these two areas, the resulting neotropical fauna has occupied
territory as far as the environmental similarity extends. This is roughly
as far as the climate can be called "tropical" in broad terms, and the
line [[p. 386]] conventionally bounding the
Neotropical Region to the north has been drawn at the equally conventional
boundary of tropical climates, delimiting an area extending to and stopping
with the hot Mexican lowlands. This is, indeed, merely a critical line
within a broad transitional zone and not a localized barrier. Few distributions
stop precisely at the line. Starting far north of it, say in Colorado
or New Mexico, and

TABLE III--FAMILIES OF RECENT LAND MAMMALS IN
SELECTED PARTS OF NORTH, CENTRAL, AND SOUTH AMERICA

progressing to regions far south of it, say to Ecuador or the Guianas,
animals mainly neotropical in distribution rather steadily become more
frequent, and those mainly nearctic less frequent. In detail, Central
America also has local differentiation, with numerous species and some
genera (but no higher categories among mammals and few among other animals)
confined to that area.

The place of Central
America within the broad outlines of static recent [[p.
387]] zoogeography is well displayed by comparison of the mammalian
families of New Mexico, Costa Rica, and the Guianas, each area with the
same number of families (twenty-one). About half the families (ten fully
and two more marginally) are common to all three areas but generally with
different species and sometimes with different genera in each. Six families
occur in New Mexico only, as among these three areas, and three in the
Guianas only. These are the most definitely nearctic and neotropical groups,
respectively. Costa Rica has no families that do not occur either in New
Mexico or in the Guianas. It has three families in common with New Mexico
but not the Guianas and six with the Guianas but not New Mexico, and is
thus intermediate in this respect but somewhat more like the Guianas,
justifying inclusion in the Neotropical Region (see Table III).

Fig. 10. Diagram of middle America as a faunal filter.
The examples given are from the recent fauna of land mammals. For some
groups the action of the filter was different in the Pleistocene.

Middle America may be
considered statically as a transition zone and historically as, successively
or intermittently, a barrier and a migration route, but these simple and
usual characterizations hardly begin to express its true role and importance.

As a transition zone
and migration route, the role is not merely that of a habitat and a pathway.
Middle America is a faunal filter (see Fig. 10). Its ecological characteristics,
in the broadest sense, determined which stocks were involved in faunal
interchanges between North and South America and which are now immobilized
to north and to south. The filtering action is not sharply localized.
It begins well to the north (and west), roughly at the edge of the lower
Sonoran life zone in southwest United States, and also reaches far to
the south and east, more or less to the edge of the Guiana highlands and
thence southward and westward. From these quite indefinite outer edges,
the filtering action becomes denser toward the central critical line,
which now [[p. 388]] approximates the border
of the central Mexican plateau. It probably had about the same or at times
a more southerly position in the Pleistocene, but in parts of the Tertiary
it may have been considerably farther north.

At present some northern
groups, such as the shrews, pocket mice, and pocket gophers, penetrate
well into the filter zone, but not beyond its vague southeastern edge.
Others, such as the bobcats, bison, and sheep, have stopped near its attenuated
northern border. For other northern forms, the filter zone has been essentially
an open passage: rabbits, squirrels, dogs, otters, pumas, and many others
among recent animals, and also horses, mastodons, and some other fairly
recently extinct groups. Most of the old southerners, South American mammals
of the first and second faunal strata, are now stopped somewhere within
the zone: most armadillos, all anteaters and tree sloths, all the old
native South American rodents except the porcupines. One armadillo (Dasypus
novemcinctus) is peculiar in that a generation or so ago it stopped
at about the northern edge of the filter zone but in the last few years
has spread well beyond this. A related form (Dasypus bellus,
now extinct) was even more widespread in North America in the Pleistocene,
when there were also a few other old southerners (glyptodonts, ground
sloths, capybaras) unaffected by the filter. It is also interesting that
the filter was in some cases permeable for ancestral forms but now completely
separates their differentiated northern and southern descendants, for
instance in the cases of the porcupines and the bears.

Most peculiar in this
respect are the rather numerous animals clearly of ultimate northern origin
and yet now stopped in or south of the filter zone: coatis, kinkajous,
the numerous kinds of South American cats other than the puma, short-faced
bears, tapirs, peccaries (reaching the extreme northern edge of the filter,
but hardly beyond), camels, and other lesser differentiates. Superficially
one can say simply that such groups happened to become extinct in North
but not in South America, a statement but not an explanation. For most
of them, at least, there is a more explanatory probability: the particular
populations and lines involved were native to Middle America, adapted
to environmental conditions prevailing there and over much of South America
and not adapted to or immediately derived from North America
above the present filter zone. In some cases there are complications requiring
modifications of or additions to this general theory (for instance, for
the bears, paramo tapirs, and camels), but even these need not be flatly
exceptional or contradictory, and for most groups involved the theory
is a simple, elegant, and sufficient explanation of their peculiar distributional
history.

It is an obvious, but
nevertheless frequently unappreciated, fact that immigrants from North
to South America did not come from the continent as a whole or from its
broad, now temperate zone, best known to us both paleontologically and
neontologically, but only from Central America. With local and geologically
brief interruptions for part of it, Central America has been continuous
with the North American land mass throughout the Cenozoic. Marine barriers
were mostly in the [[p. 389]] extreme southeastern
part of Central America and the major barrier, especially toward the end
of the Cenozoic, was in what is now part of South, not Central, America
(in western Colombia). Thus Central or broadly Middle America must have
had a fauna mainly or purely North American in origin and in broad faunal
type until toward the end of the Pliocene, a conclusion usefully attested
by the one known Tertiary land fauna from Central America, early Pliocene
in age and entirely North American in affinities. Nevertheless, Middle
America must have been an important center of regional faunal differentiation
within the North American general fauna.

Even in the early Cenozoic,
when climatic zoning was less sharp than now, such zoning did exist, and
Middle America has by astronomical and meteorological necessity always
been the warmest (or most evenly warm) and tropical part of North America.
It must long have been a center of adaptation and local radiation of faunal
elements specifically adapted to its special conditions, not only climatic
but also edaphic, floral, etc. This special local fauna was the one, and
the only one, available for spread to South America, in all of the faunal
strata of the latter region. Some Central American differentiates have
succeeded in spreading northward (jaguars in the Pleistocene, for instance,
and probably a number of otherwise mysterious newcomers in more northern
fossil fields throughout the Cenozoic). Spread southward, when geographically
possible, was more general and rapid because northern South America is
more like Middle America in climate and associated factors. The Tertiary
sea barrier did not follow a climatic zonal boundary, and when connection
was established the incorporation of Central America into the South American
and of South America into the Central American faunal zone was rapid and
reciprocal.

Thus may be readily explained
neotropical differentiation within many groups of old northern origin,
the roots of which have not been found among the fossils from higher latitudes
in North America. The more distinctively neotropical forms (at levels
mainly of specific to generic differentiation) in many families may be
inferred to have been Middle American differentiates: numerous noncaviomorph
rodents, some dogs, the southern procyonids (sensulato),
most of the southern cats and most of the deer, among others. There is
also discernible, on these grounds, a minor stratification within
the broad late stratum of the South American fauna, between forms which
had been longer in Middle America and more differentiated there, and those
newer or less isolated in that region. Thus the coatis (Nasua)
are inferred to be older and their allies the true raccoons (Procyon)
younger Middle American forms, or among the deer the brockets (Mazama)
may be older and the closer allies of our white-tails (Odocoileus)
younger there. In some such cases there are other factors to consider,
especially the possibility and timing of spread northward from Middle
America. Here emphasis can only be placed on broader aspects of the historical
role of Middle America and details cannot further be discussed. In fact,
these details have not yet been adequately studied, if at all, from this
point of view.